Soils And Crop Changes

Peas and sweet peas do not grow well continuously in the same ground. I
know this practically in my experience, but in no book have I ever found
why they do not grow.

There are two very good reasons why some classes of plants cannot be
well grown continuously in the same piece of ground. One is the
depletion of available plant food, the other the formation of injurious
compounds by the plants, or the gradual increase of fungoid, bacterial
or animate pests in the soil, which finally become abundant enough to
seriously hinder growth. Different plants take the plant foods, as
nitrogen, lime, potash, phosphates, etc., in different proportion. More
important, perhaps, is the fact that the root acids that extract these
foods are of different types and strength. Thus before many seasons it
may happen that most of the plant food of one or more kinds may be
nearly exhausted as far as that kind of plant is concerned that has
grown there continually, while there would be plenty of easily available
food for plants with a different kind of root system and different root
acids, etc. This is one reason why rotation of crops is so good; it
gives a combination of root acids and root systems to the soil during a
term of years, and it also frees the soil from one certain kind of
organism because it cannot survive the absence of the particular plants
on which it thrives.